Slags of non-ferrous metallurgy are generated as by-product of smelting concentrate or converting matte to separate metal fraction from unwanted fractions. Slag is primarily a mixture of metal oxides and silicon oxides, but it can also contain metal sulfides and metals in elemental form.
By way of example, slag tapped off from a copper flash smelting furnace may contain, depending on the raw material, for instance, magnetite, fayalite, zinc, copper, lead, arsenic, cadmium, and nickel. At present the slag is cleaned either by reduction in an electric furnace or using slag concentration technology. After this kind of cleaning, the slag still contains, depending on the treatment and the raw material, about 0.3-1% copper, about 1-4% zinc, about 0.1-0.4% lead and about 0.1-0.2% arsenic. Such copper and zinc contents in the slag are economically seen quite a loss. Furthermore, the waste slag received from a slag concentrator is very fine, with a grain size of under 0.125 mm. Therefore, detrimental substances contained in the slag may leach when on a dump, thus generating an environmental threat.
It is very common that the waste slag still contains valuable metals and detrimental substances, which tend to make the slag a problem waste unsuitable for utilization. Dumping such slag is expensive because a dumping area must have dense foundation and because storing may require long-term monitoring.
Often the aim of the slag cleaning process is to maximize the recovery of valuable metals such as Co, Ni, and Cu in an alloy with the lowest possible iron content. The amount of metallic iron produced should be kept to a minimum, as the more iron is present in the resulting matte or alloy, the greater the costs of the subsequent hydrometallurgical separation of the valuable metals and the resulting disposal of iron residues.
Generally, the purpose of cleaning copper containing slag in an electric furnace is to reduce oxidized copper to metallic copper and trivalent iron to divalent iron and to settle the metallic copper droplets from the slag, thereby forming a metal layer beneath a slag layer. As the oxygen potential of the slag decreases further, also further reductions take place, such as reduction of divalent iron to metallic iron and reduction of oxidized lead to metallic lead. Stirring the material in the electric furnace can be used to intensify the reduction reactions.
Processes for cleaning slags of non-ferrous metallurgy by reduction in an electric furnace have been presented, for instance, in FI 84368 B, U.S. Pat. No. 4,717,419 A, U.S. Pat. No. 5,332,414 A and U.S. Pat. No. 5,411,572 A. In all these processes reduction is carried out as a partial reduction; in other words, reduction is terminated before metallic iron starts to form. At this stage there is still some copper left in the residual slag. Furthermore, zinc, lead, cadmium and arsenic have not yet completely vaporized. Such reduction is often carried out by surface coke reduction, which requires a long time, because the metal droplets formed during the reduction must be settled, forming a layer of molten metal below the layer of molten slag.
If slag reduction in the electric furnace is carried on further, also iron starts to reduce, and metals with a low boiling point, such as zinc, lead, cadmium and arsenic, vaporize. According to WO 2009/077651 A1, it is known from the prior art to reduce the slag from a suspension smelting furnace in an electric furnace so far that, after the slag reduction, the slag's copper content is so low that a further treatment of the waste slag obtained from the electric furnace is not economically feasible.
U.S. Pat. No. 8,088,192 B2 discloses a three-phase process for recovering non-ferrous metals from metallurgical residuals. The process comprises: (A) a fusion and reduction phase during which a certain quantity of iron is reduced and passes into a copper bath; (B) a settling phase during which metal droplets are allowed to settle from the slag to the copper bath and a part of the slag is removed from the furnace; and (C) an oxidation phase involving oxidation of the iron in the copper bath. Certain non-ferrous compounds are volatilized during phase A and carried away by fumes. Volatile heavy metals, in particular zinc and lead, are recovered from the fumes by means of separators. The reference also teaches stirring the copper bath by injection of inert gas in an alternating current plasma arc furnace treating metallurgical residues. The process is complicated, takes a lot of time and requires large size of reduction furnace.
Another way to improve reduction in an electric furnace comprises introducing inert gas through porous plugs mounted on the bottom of the furnace.
Prior art contains various kinds of substances that can be used as reducing agents in slag reduction. Just to mention a couple of examples: WO 20060240069 teaches the use of carboniferous polymers as metal oxide reducing agents in ferroalloy production; DE 19541673 A1 teaches using ground plastic as a reducing agent in a shaft furnace; in Isasmelt™ reactors, coke may be replaced with plastic.
Consequently, there are lots of reducing agents that can be used in slag reduction, and there are numerous ways to enhance reduction in an electric furnace, for instance, by mixing. However, in the processes of the prior art, the mixing step must always be followed by a settling step to allow the separation of the metal phase from the slag. Settling of metal droplets from the molten slag phase is a slow process. Consequently, a large size of electric furnace and large amount of energy are needed to maintain the desired temperature in the furnace.